1. Field of the Invention
The present invention relates to a flow stop control method for ensuring the desired stop condition in a casting withdrawal control of a horizontal continuous casting machine.
2. Description of the Prior Art
In known horizontal continuous casting machines, the rotation of a driving hydraulic motor which is restricted by a servo valve is transmitted to the shafts of pinch rolls through a group of gears and the pinch rolls are driven by the rotation of the pinch roll shafts thereby withdrawing the casting. This withdrawal of the casting is effected through a withdrawing cycle, consisting of withdrawing, stopping and push-back periods, and during the stopping period, the flow or run of the casting is stopped by restricting the flow rate of a braking hydraulic motor connected to the gear train through a throttle valve and thereby applying a braking torque in the withdrawing direction of the pinch rolls.
Since the application of the braking torque in the withdrawing direction of the pinch rolls by the braking hydraulic motor is effected on the basis of its internal pressure restricted by the throttle valve, it is difficult to apply any given constant braking torque. In other words, the braking torque, due to the braking hydraulic motor internal pressure, is varied by the rotation speed of the pinch rolls. Therefore, the adjustment of the braking torque through the opening adjustment of the throttle valve must inevitably have recourse to experiments.
Also, this unstable braking torque is applied even during the withdrawing period and the push-back period other than during the casting flow stop in the stopping period of the withdrawing cycle the unstable braking torque is contradictory action, due to the flow stopping action during the stopping period and the load torque during the withdrawing period, and the withdrawing waveform tends to become unstable. In other words, the unstable braking torque is applied even during the withdrawing period and the push-back period after the casting flow stop in the stopping period of the withdrawing cycle. Therefore, the braking torque functions as casting flow stop during the stopping period but it also functions as load torque during the withdrawing period. When the braking torque functions as load torque, the power for withdrawing the casting inevitably varies and the withdrawing waveform tends to become unstable.
Also, while, in the horizontal continuous casting machine, the load variations (the variations of the friction and the load inertia) due to the casting condition exist to a degree that cannot be disregarded, the above-mentioned conventional flow stop control method gives no consideration to the load variations and the control is effected by use of the unstable braking torque thus making it difficult to ensure a stable stop condition. There is another disadvantage that the stopping period in the withdrawing cycle is the period of time required for the growth of a shell to be newly formed so that if this stopping period is not stably ensured, the growth of the shell is incomplete and a rupture (breaking apart) of the shell occurs during the next withdrawing step thus making it difficult to ensure the stable casting operation.